The present invention is based on patent applications Nos. 2000-173088 Pat., and 2000-173091 Pat. filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of The Invention
The present invention relates to an image display medium and more particularly to a reversible image display medium capable of rewriting the images (namely allowing formation, erasure and rewriting of images).
2. Description of The Background Art
At present, the text, graphics or the like produced by a computer, a word processor or the like is displayed on a display such as a CRT display, or is output on a medium such as a paper sheet via a printer for display.
The images displayed by a display such as a CRT display show a low resolution, and the resolution, clarity and precision of images are restricted, as compared with the images displayed by the printer or the like on the paper medium or the like. Due to the relatively low resolution and the light emission from the display, long-time viewing of displayed images may be hard to eyes.
When the text, pictures or the like is displayed on a image display medium of paper or the like, clear images can be displayed in a high resolution, and thus easy-on-the eyes display can be achieved.
For these reasons, the text, graphics or the like produced by a computer, a word processor or the like is output in most cases on a medium such as a paper sheet via a printer for display, whether when the text or the like is skimmed or drafted or when it is output after completion.
The medium of paper or the like bearing images will be abandoned or burnt when it is no longer required. This results in consumption of a large mount of resources. The printer or the like also consume large amounts of consumable products or materials such as toner, ink or heat transfer sheets. For obtaining the new display mediums of paper or the like as well as toner, ink or the like, energies and resources are required for producing them.
This is contrary to the current demand for reduction in environmental loads.
An object of the present invention is to provide a reversible image display medium which is capable of rewriting (namely which allows image formation, image erasure and image rewriting) and which, therefore, is capable of reducing the amount of consumable products or materials such as paper or like image display media, developers, and ink to be used for conventional image display, namely capable of meeting the current demand for reduction in environmental loads.
Another object of this invention is to provide a reversible image display medium which is capable of forming images with low energy, namely capable of meeting the current demand for reduction in environmental loads.
A further object of the invention is to provide a reversible image display medium which is capable of displaying images in good contrast.
A still further object of the invention is to provide a reversible image display medium which is capable of displaying images in high resolution.
An additional object of the invention is to provide a reversible image display medium which is capable of smoothly providing image display and image erasure, and image rewriting.
A still another object of the invention is to provide a reversible image display medium which is capable of stably displaying images for a long time.
This invention provides the following first to fourth types of reversible image display media.
 less than First Type Reversible Image Display Medium greater than 
A reversible image display medium comprising:
two substrates opposed to each other with a predetermined gap therebetween;
one or more developer-accommodating cells formed between the two substrates; and
a dry developer accommodated in each cell, wherein
the dry developer contains at least two kinds of frictionally chargeable dry developer particles having different chargeable polarities and different optical reflection densities, at least one kind of the dry developer particles being magnetic particles containing a binder resin having a magnetic substance incorporated therein, the magnetic particles having a saturation magnetization degree in the range of 20 emu/g to 50 emu/g.
 less than Second Type Reversible Image Display Medium greater than 
A reversible image display medium comprising:
two substrates opposed to each other with a predetermined gap therebetween;
one or more developer-accommodating cells formed between the two substrates; and
a dry developer accommodated in each cell, wherein
the dry developer contains at least two kinds of frictionally chargeable dry developer particles having different chargeable polarities and different optical reflection densities, at least one kind of the dry developer particles having a volume resistivity xcfx81 in the range of 1.0xc3x971010 xcexa9xc2x7cm to 1.0xc3x971014 xcexa9xc2x7cm.
The reversible image display media of the invention include reversible image display media having the features of both the first and second types of reversible image display media.
 less than Third Type Reversible Image Display Medium greater than 
A reversible image display medium comprising:
two substrates opposed to each other with a predetermined gap therebetween;
one or more developer-accommodating cells formed between the two substrates; and
a dry developer accommodated in each cell, wherein
the dry developer contains a fluidization agent in the form of an inorganic fine powder, the inorganic fine powder having a BET specific surface area of 80 m2/g to 300 m2/g.
 less than Fourth Type Reversible Image Display Medium greater than 
A reversible image display medium comprising:
two substrates opposed to each other with a predetermined gap therebetween;
one or more developer-accommodating cells formed between the two substrates; and
a dry developer accommodated in each cell, wherein
the dry developer is accommodated in the cell in an amount of 20 vol. % to 50 vol. % based on the volume of the cell.
The reversible image display media of the present invention include reversible image display media having the features of both the third and fourth types of reversible image display media.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.